In communication and control systems, delay lines are used to store a signal for a discrete period, and to supply that signal at an output point at the end of the period. This period between the time the signal is input and the time the signal is output is called delay time. Fiber optic switches, particularly for multi-mode fibers, are known as useful devices as delay lines. For example, transversal filters capable of selectively filtering modulated light signals are known. These filters are constructed by helically wrapping a single fiber optic element around a series of v-grooves in a silicon chip with taps at each groove. A problem with these types of delays is that once the devices are constructed, no adjustment of the delay line lengths may be made and, hence, once constructed, the frequency versus attenuation and, consequently, the delay characteristic, as set, is unalterable. Desirable would be a fiber optic discretely variable delay line capable of providing an incrementally adjustable delay to an optical transmission system. With coupling of such fiber optic discretely variable delay line, a variable incremental delay could be imparted to a conventional electrical transmission line.
The present invention relates to an optical fiber switch capable of imparting a discrete delay in transmission of an optical signal as compared to the transmission of a base signal. Further, the present invention relates to a fiber optic discretely variable delay line comprising one or more of said switches.
Gutterman, et al, U.S. Pat. No. 4,854,660, relates to a particular type of fiber optic switch based on the imaging property of a spherical reflecting surface such as that disclosed by Kokoshvill, U.S. patent application Ser. No. 053,220, entitled "Fiber Optic Bypass Switch" filed on May 13, 1987, and further identified by priority publication number EP0299604 Al, the contents of this application incorporated into the Gutterman, et al patent by reference. As taught therein, a point source of light slightly displaced from the center of curvature of a spherical reflector is imaged with minimal aberration at a point symmetrically located with respect to the center of curvature. Switches taught by the Kaptron patent application and the Gutterman patent involve rotating a spherical reflector between first and second positions relative to an array of optical fiber end faces. Illustratively, there is an optical fiber end face (T) which terminates a fiber connected to the transmitter of a node and an optical fiber end face (R) which terminates a fiber connected to the receiver of the node. Two additional fiber end faces (I and O) terminate the incoming fiber and the outgoing fiber respectively. There are also two other optical fiber end faces (L1 and L2) which correspond to the first and second ends of a fiber optic loop.
In the first reflector position, the T and O end faces are conjugate (i.e. symmetrically located with respect to the center of curvature) and the I and R end faces are conjugate. Accordingly, light emanating from the input fiber end face (I) is imaged by the spherical reflector into the receiver fiber end face (R) and light emanating from the transmitter fiber end face (T) is imaged into the output fiber end face (O). Thus in its first position, the switch of the present invention can be used to insert a node into a fiber optic network by coupling light from the incoming fiber to the receiver and light from the transmitter into the outgoing fiber.
In the second reflector position, the input and output fiber end faces (I and O) have conjugate locations so that light from the incoming fiber is now imaged by the spherical reflector into the outgoing fiber instead of the receiver so that the node is bypassed. Simultaneously, in the second reflector position, the transmitter fiber end face (T) and the first fiber loop end face (L1) are conjugate and the receiver fiber end face (R) and the second fiber loop end face (L2) are conjugate. Thus, light emanating from the transmitter fiber end face (T) is imaged by the reflector into the first fiber loop end face (L1). This light enters the fiber loop and emerges at the second fiber loop end face (L2). Light emanating from the second fiber loop end face (L2) is imaged by the reflector into the receiver fiber end face (R). Thus the transmitter and receiver are connected to each other by an optical path enabling the bypassed node to be tested. The fiber optic loop used in the testing path between the transmitter and receiver provides a suitable amount of attenuation so that the receiver is not saturated when the node is tested.
The operation of the switch of the above identified patent application may be summarized as follows. Light arrives via a first optical fiber (the incoming optical fiber) and may be imaged by a spherical reflector into a second optical fiber chosen from a plurality of available fibers (the receiver fiber or the outgoing fiber) depending on which of two positions the reflector is pivoted.
The Gutterman, et al patent discloses a type of switch as taught by the Kokoshvill patent application and relates to a switch for selectively coupling optical fibers wherein the switch includes an imaging system, having a symmetry such as a spherical reflector. The switch also includes a group of optical fiber end faces including a first optical fiber end face via which light is transmitted to the imaging system and at least second and third optical fiber end faces. A translation mechanism is provided for linearly translating the imaging system and the fiber end face group relative to one another between two positions. In a first position, the first and second fiber end faces are conjugate with respect to the symmetry of the imaging system so that light from the first fiber is imaged by the imaging system into the second fiber. In a second position, the first and third fiber end faces are conjugate with respect to the symmetry of the imaging system so that light from the first fiber is imaged by the imaging system into the third fiber. Thus, it is possible to switch the light from the first fiber into the second fiber or into the third fiber depending on the position of the linear translation mechanism.
In another embodiment as taught in the Gutterman, et al. patent, the switch comprises the imaging system having a symmetry and a group of six optical fiber end faces. Light is transmitted to the imaging system via the first optical end face and the fourth optical end face. The fifth and sixth optical end faces are the end faces of an optical fiber loop. The switch includes means for linearly translating the group of fiber end faces and the imaging system relative to one another from a first and a second position. In the first position, light from the first fiber end face is imaged into the second fiber end face and light from the third fiber end face is imaged into the second fiber end face and light from the fourth fiber end face is imaged into the third fiber end face. In the second position, light from the first fiber end face is imaged into the third fiber end face and light from the fourth fiber end face is imaged into the fifth fiber end face. The light imaged into the fifth fiber end face is propagated through the fiber loop to the sixth fiber end face and then imaged into the second fiber end face.
In accord with the present invention, it has been found that a double loop system structurally enables a precise and discrete delay to be provided by a switching mechanism utilizing the fundamental principles of the Gutterman, et al. and Kokoshvill references. It has further been found in the present invention that the switches which act as delays as defined by the present invention may advantageously be incorporated into a fiber optic variable discrete delay line wherein a selected combination of switches impart the capability to provide a selected discrete delay in transmission of an optical signal.
Switches of a 2.times.2 configuration with a single loop are known. However, these switches are unsatisfactory for use in a delay system because of differing attenuation in respective paths. Thus, in one position, say an "off" position, a signal arrives at the switch, passes therethrough and then exits with little comparative attenuation, while a signal arriving at the switch in an "on" position transfers to the fiber optic loop, through the loop and then is transferred to the output fiber with a loss comparatively greater than that as in the first condition. It has been found with the present invention that by providing two loops, regardless of whether the signal passes through the "through" loop or through the "delay" loop, comparative attenuation, i.e. loss, substantially remains constant.
The loops of the switches of the present invention may be formed and carried within the body of the switch as is preferable with either the switch of the Gutterman, et al patent or the switch of the Kokoshvill patent application, or the loop may be formed outside the body of the switch, for example, as with the switch taught by Lee, U.S. Pat. No. 4,834,488. In one embodiment taught by Lee, an optical switching device comprises a first and second fixed optical fiber along with a fiber end, a rotatable member and third and fourth optical fibers and second fiber end attached to the rotatable member. The first and second fiber ends are connected by a fiber loop. The member is rotatable between two switching positions so that when the member is rotated to one position, the third and fourth moveable fibers are substantially optically aligned with the first and second fixed fibers respectively. When the member is rotated to the next switching position, the third moveable fiber is substantially aligned with the second moveable fiber and the fourth moveable fiber is substantially aligned with the first end face and the first moveable fiber is aligned with the second end face. With the modification of the present invention, an additional loop is formed between one of the two of the first and second fibers and one of the two of the third and fourth fibers.
Further, the present invention relates to an optical time delay device for electrical signals such as that taught by Soref, U.S. Pat. No. 4,671,605, which teaches a length dependent optical time delay/filter device for electrical signals comprising a single integrated optical switching circuit and only two optical time delay components being optically connected to each other through a switching circuit. The length dependent optical time delay/filter device includes means interconnected by a single mode optical fiber to the integrated optical switching circuit for receiving an optical input signal and converting the input signal into an optical signal. The optical signal is received a by preselected optical fiber of one optical time delay component and, thereafter, by a preselected fiber of other optical time delay components. The switching circuit determines which preselected fibers the optical signal passes through. The length dependent optical time delay/filter device further includes means for receiving the optical signal as it outputs from the other optical time delay component and for converting the output signal to an electrical signal. The present invention relates to an optical time delay device utilizing the improved optical fiber switches comprising an imaging system and grouping of fiber optic end faces as hereinafter described.